Chromium is an unavoidable trace element of the raw material used in the manufacture of cement clinker. The oxidizing and alkaline burning conditions of the cement kiln forms toxic hexavalent chromium (CrVI). Hexavalent chromium is a powerful dermal irritant considered to be extremely toxic due to its high oxidation potential and ability to penetrate human tissue. It can cause skin sensitization, allergic reactions, and eczema. Chromium VI has high solubility in water and is released when cement is mixed with water. Thus, wet cement poses a health issue for workers who come into contact with wet cement or concrete.
Currently, the typical approach is to employ ferrous (Iron II) sulfate to reduce hexavalent chromium (CrVI) to trivalent chromium (CrIII), which tends to precipitate from solution, thereby posing a much reduced risk as a dermal irritant. However, the use of ferrous sulfate is not efficient because the dosage amount required for reducing CrVI to CrIII is at least ten times the stoichiometric amount of ferrous sulfate otherwise required. The poor dosage efficiency is explained partly by the fact that ferrous sulfate readily oxidizes from Iron (II) to Iron (III) during prolonged contact with air and water. This poor storage stability decreases the efficiency of ferrous sulfate, and can render it useless as a chromate reducer if not used quickly.
The fact that dry Iron (II) salts can be oxidized to Iron (III) with prolonged contact in water precludes it from being dispensed in a water solution.
Moreover, dry Iron (II) can not be added to cement over 80° C., without risking considerable oxidation. Since cement exits the mill at temperatures up to 130° C., and goes to storage at temperatures in excess of 80° C., the cement plant is limited to using excessively high doses of iron (II) sulfate very late in their manufacturing and distribution process. It is often added just prior to packaging in bags. This necessitates additional quality control monitoring steps to assure CrVI minimum level compliance. This is both inconvenient and costly for the cement plant operator. In addition to the extra cost and inconvenience, an excessive level of iron sulfate can be detrimental to cementitious systems due to increased water demand and extended set time.
As an alternative to iron (II) sulfate, stannous (tin 2+) salts can be employed as chromium reducers. The required dosages of solid stannous salts (˜0.02% Sn2+ ion based on the weight of cement) are much lower than that of iron II sulfate. The present inventors believe that stannous salts are easier to use and would be more heat resistant and storage stable when combined with cement in the milling process. Although stannous sulfate is water soluble, it quickly loses dosage efficiency when added as an aqueous solution to cement. The actual amount of stannous sulfate needed in solution is at least double the amount that is required when stannous sulfate is added as a powder. Such a disparity has precluded the use of stannous sulfate in solution form.
In view of the foregoing prior art disadvantages, there exists a need for a novel chromium reducer, which can be added as a low dosage liquid material, and which is storage and heat stable.